Microstructural evolution and growth velocity-undercooling relationships in the systems Cu, Cu-O and Cu-Sn at high undercooling

A melt encasement (fluxing) technique has been used to systematically study the velocity-undercooling relationship in samples of Cu and Cu-O and Cu-3 wt% Sn at undercoolings up to 250 K. In pure Cu the solidification velocity increased smoothly with undercooling up to a maximum of 97 m s(-1). No evi dence of grain refinement was found in any of the as-solidified samples. Ho wever, in Cu doped with > 200 ppm O we found that samples undercooled by mo re than 190 K had a grain refined microstructure and that this corresponded with a clear discontinuity in the velocity-undercooling curve. Microstruct ural evidence in these samples is indicative of dendritic fragmentation hav ing occurred. In Cu-Sn grain refinement was observed at the highest underco olings (greater than 190 K in Cu-3 wt% Sn) but without the spherical substr ucture seen to accompany grain refinement in Cu-O alloys. Microstructural a nalysis using light microscopy, texture analysis and microhardness measurem ents reveals that recrystallisation accompanies the grain refinement at hig h undercoolings. Furthermore, at undercoolings between 110 K and 190 K, a h igh density of subgrains are seen within the microstructure which indicate the occurrence of recovery, a phenomenon previously unreported in samples s olidified from highly undercooled melts. (C) 2000 Kluwer Academic Publisher s.